1. Center for Molecular Medicine and Infectious Diseases, Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
2. Present address: Department of Pathology, University of Florida School of Medicine, Gainesville, FL 32610.
3. Division of Infectious Diseases and Geographic Medicine, Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, and Department of Immunology and Infectious Diseases, Research Institute, Palo Alto Medical Foundation, Palo Alto, CA 94301.
4. Present address: Department of Internal Medicine, Ageo Kosei Hospital, Ageo-Shi, Saitama, Japan.